The dark, faint galaxy Dragonfly 44 from afar (left) and close-up (right). The galaxy has a large, elongated shape, and is surrounded by a halo of star clusters. similar to the halo that surrounds our Milky Way Galaxy. Credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA.
When you hear the word “galaxy,” you likely imagine the gauzy arms of the Milky Way, teeming with cheery, glowing stars as it merrily swirls in space.
But not all galaxies are so revered as our own that they appear on everything from clothes to mugs. A paper published in Astrophysical Journal Letters this week revealed a giant galaxy whose mass is mostly dark matter — a “failed” galaxy devoid of star formation that’s as grim as it is sparsely populated.
In other words, perfect ingredients to make a supervillain.
One of its advantages: near-invisibility. The Dragonfly galaxy has just 1 percent of the stars that the Milky Way does, and it qualifies as an ultra diffuse galaxy, or UDG. UDGs can be as big as the Milky Way, but they’re extremely faint due to their lack of brightness.
The Coma Cluster, a massive gathering of galaxies located towards the constellation of Coma Berenices. Credit: ESA/Hubble & NASA; D. Carter (LJMU), Acknowledgement: Nick Rose
Turns out, what the UDGs lack in light, they make up for in heft. Hulky, terrifying heft. Despite appearances, the Dragonfly galaxy’s mass was unveiled to be a trillion times that of the sun — comparable to the Milky Way’s.
The galaxy has only been on the cosmic radar for a few years. In 2013, astronomers and study authors Pieter van Dokkum and Roberto Abraham built a very small new multi-lens array called Dragonfly, which could detect very dim, complex structure around galaxies. It even looks like a dragon’s compound eye.
They interrupted their initial survey to peer at a cluster called Coma, and Dragonfly’s eight Canon lenses unexpectedly showed nearly 50 faint, fuzzy blobs the size of the Milky Way. These blobs didn’t resemble classic low surface brightness galaxies. These were different: darker, redder. The authors proposed the term ultra diffuse galaxy, and it stuck.
Soon, hundreds of more UDGs emerged.
But a question lingered: If the stars in each galaxy were so sparse, what was the space glue keeping the whole shebang together? Shouldn’t the chaos of the cosmos naturally shred those stars apart like taco meat?
“Motions of the stars tell you how much matter there is,” van Dokkum said in a statement. And so the international team turned to the stars’ velocities to deduce just how big this ominous twin might be. Zoning in on Dragonfly 44, the largest (and second-brightest) in their sample, the researchers utilized the Keck II telescope of Keck Observatory. A spectrometer from the Gemini North telescope in Hawaii also chipped in some observations.
Using both small and gigantic telescopes, not only did the team glean the galaxy’s mass, it also located almost a hundred globular clusters around the galaxy — more evidence that the galaxy is a beast.
Why Dragonfly 44 failed to evolve into a cheery galaxy is unknown. It’s possible that something stole the gas from its stars, or that the entire thing got pulled off like taffy from another galaxy.
Or maybe it’s just an antihero that’s a little sad that it’ll never get its own galactic leggings.